The current pig-iron smelting mainly employs the blast furnace technology, which requires uses of coke and pellet, involving a relatively long workflow. Non-blast furnace ironmaking technologies have been paid more and more attention for lessening the dependence on coke. Non-blast ironmaking technologies can be divided into two types: direct reduction and melting reduction. The product from direct reduction is sponge iron obtained from the reduction of iron ore concentrate and mainly used as raw material for electric furnace steelmaking. In melting reduction, the sponge iron is further melted so as to obtain liquid iron after separation of slag-iron. Both direct reduction and melting reduction need to undergo a gas-solid phase reduction process of the iron ore concentrate. Their difference mainly lies in the metallization ratio. Direct reduction typically requires a metallization ratio of more than 90%, whereas the metallization ratio for the gas-solid phase reduction (commonly known as pre-reduction) in melting reduction can be as low as 75% due to the presence of subsequent final reduction in a molten bath. The pre-reduction of both direct reduction and melting reduction involves basically the same processes, during which solid iron ore is reduced in a gas phase. Therefore, they are collectively referred to as iron ore reduction in the present application. There are many methods for reducing iron ore, which can be divided into different categories, such as rotary kiln, rotary furnace, shaft furnace, fluidized bed, etc., according to the types of reduction reactors. The fluidized bed reduction reactor is recognized as the most efficient iron ore reduction reactor due to its prominent advantages including direct processing of powdered ore, good heat and mass transfer, high reduction efficiency, etc. compared to other reactors. Reduction of iron ore on a fluidized bed has been investigated for several decades, and a large number of process patent applications have been filed and some technologies such as FIOR/FINMET, FINEX, HISmelt, Circofer, and Circored have undergone pilot scale tests or been industrialized.
The FIOR process was first studied by the ESSO Research and Engineering Company in the 1950s. A 5t/d laboratory-scale trial was completed in ESSO's laboratory (Baton Rouge, La., USA) in 1962, and a 300t/d factory was established in Canada (Darmouth, Nova Scotia) in 1965. In 1976, an industrial plant for producing hot briquettes with an annual output of 400,000 tons (U.S. Pat. Nos. 5,082,251 and 5,192,486) was established in Venezuela, and named as FIOR from the initials of “Fine Iron Ore Reduction”. The plant has been putting into operation continuously till today. Having been developing the FINMET technology in cooperation with Voestalpine since 1992, the FIOR Company completed the validation for the FINMET technology in 1995. A FINMET system with an annual output of 500,000 tons began to be established in January 1998, was debugged in November 1999, and was put into operation formally in May 2000. Since 2001, two FINMET systems having each an annual output of 500,000 tons have been merged into a system with an annual output of 1 million tons. H2+CO obtained from natural gas reforming is used as the reducing and fluidizing medium in FIOR/FINMET, in which four stages of fluidized beds are operated in series. The reducing gas with a gas pressure of 11-13 atm (gage pressure) passes through the fourth fluidized bed, the third fluidized bed, the second fluidized bed and the first fluidized bed in series. The unconsumed H2 and CO in reduction exhaust gas are recycled after a purification process such as dust removal, decarbonization, etc. The iron ore powder is reduced on the first fluidized bed, the second fluidized bed, the third fluidized bed and the fourth fluidized bed in series (Schenk, et al., Particuology, 2011, 914-23). As there is no intermediary heat supplementation, the temperatures of the fluidized beds are decreased gradually from the fourth fluidized bed with a reduction temperature of about 800° C. to the first fluidized bed with a reduction temperature of only 400-500° C., at which temperature the reduction rate is so low that the first fluidized bed is mainly used for preheating.
The FINEX process is a new melting reduction process based on pre-reduction on a fluidized bed, which was developed by Pohang Iron and Steel Co. Ltd, Korean in cooperation with Voestalpine on the basis of the COREX melting reduction technology of Voestalpine. The technical development of FINEX was started in 1992; a 15t/d laboratory scale-up experiment was completed in 1996; a 150t/d pilot scale test was completed in 1999; a demonstration project with an annual output of 800,000 tons began to be constructed in January 2001, reaching the target output in May 2004; and an industrialized demonstration plant with an annual output of 1500,000 tons was established in May 2007. The pre-reduction part of the FINEX process (U.S. Pat. Nos. 5,762,681, 5,785,733, CN95191907.5, CN95191873.7, US20020166412, US20060119023, US20080302212, and US20080277842), which is basically the same as that of the FIOR/FINMET process, is operated using four-stage fluidized beds in series, except that the clean coal gas obtained through purification of the coal gas generated from melting reduction (in a melting reduction furnace) is used as the reducing and fluidizing medium. The reducing gas with a gas pressure of 2.3-4.0 atm (gage pressure) passes through the fourth fluidized bed, the third fluidized bed, the second fluidized bed and the first fluidized bed in series. The unconsumed H2 and CO in reduction exhaust gas are recycled after purification processes such as dust removal, decarburization, etc. The iron ore powder is reduced while passing through the first fluidized bed, the second fluidized bed, the third fluidized bed and the fourth fluidized bed in series. Like FIOR/FINMET, no separate iron ore powder preheating unit is set up in FINEX. The first fluidized bed mainly serves for drying/preheating at a temperature of about 400° C., and the fourth fluidized bed is at a temperature of 800-900° C. The fluidized bed reduction in the FINEX process has been in operation for years with an annual output of 1500,000 tons (the actual output of reduced iron ore is expected to exceed 2700,000 tons per year).
The Circofer process and Circored process are iron ore reduction processes on coal-based and gas-based fluidized beds, respectively, developed by Lurgi, Germany (Lurgi has sold its metallurgical business to Outokumpu, Finland). In the Circored process (U.S. Pat. No. 5,603,748, and Schenk et al., Particuology, 2011, 914-23), hydrogen is used as the reducing medium, and two-stage fluidized beds in series are employed for reduction. First, the iron ore powder is dried and preheated to 850-900° C. in a preheating unit (a combined transferring fluidized bed-cyclone-venturi preheater), and then enters into a circulating fluidized bed at a temperature of 850-900° C. for pre-reduction (first-stage reduction). The pre-reduced iron ore powder discharged from the circulating fluidized bed enters into a bubbling fluidized bed for final reduction (second-stage reduction). The bubbling fluidized bed is a horizontal transverse multi-sectional (multi-staged) fluidized bed with an operational temperature of 630-650° C. The operational pressure of both fluidized beds is 4 atm (gage pressure). The reduced iron ore powder discharged from the bubbling fluidized bed is heated by a rapid heater to a temperature above 680° C. as required for briquetting, and enters into a hot briquetting section for being briquetted. The reduction exhaust gas is recycled after treatments such as exchanging heat with circulating gas for sensible heat recycling, purification, compression, etc. Lurgi began to study the technology of iron ore reduction on circulating fluidized beds in the 1970s, primarily focused on the development of the technology with coal as the reducing medium from 1973 to 1990, and later shifted to research on the reduction technology on fluidized beds with hydrogen as the medium in 1993. In 1996, the company began to establish a demonstration project with an annual output of 500,000 tons in its plant in Trinidad, the construction of which was completed in March 1999. In May 1999, the first batch of hot briquetted iron was obtained. From 1999 to 2001, the system was debugged, optimized and restructured. In August 2001, a targeted goal of 63.6t/h of HBI was successfully achieved. From August to November in 2001, about 130,000 tons of HBI was produced continuously. However, the system has been shut down for market reasons since November 2001. The Circofer process (US20070256519, CN100587080C, and CN100540698C) is a coal-based fluidized reduction process of iron ore developed by the Lurgi/Outokumpu company (Orth, et al., Minerals Engineering, 2007, 854-861). In this process, the iron ore powder exchanges heat with the exhaust gas discharged from the first fluidized bed in a combined cyclone-venturi heater. After being preheated, the iron ore powder enters into the front chamber of the first fluidized bed; meanwhile, coal powder is added and oxygen is introduced into the front chamber. The iron ore powder is preheated by the heat generated from partial combustion of the coal powder, during which reductive gas is generated at the same time. The preheated iron ore powder and the generated gas enter into the main bed of the first fluidized bed from the top of the front chamber of the first fluidized bed. The clean coal gas obtained through purification of the exhaust enters into the bottom of the first fluidized bed as the fluidizing and reducing medium. After being reduced, the iron ore powder is discharged from the lower part of the first fluidized bed into the second fluidized bed for subsequent reduction. The sponge iron obtained from the reduction process is discharged from the second fluidized bed, passes through a hot magnetic separator to remove semi coke particles contained therein, and then goes into a smelting and separating furnace for slag-iron separation. The hot exhaust gas discharged from the top of the first fluidized bed passes through the cyclone dust remover for separation, and then enters into a combined cyclone-venturi preheater to heat up the iron ore powder while cooling down the exhaust gas. After passing through a waste heat boiler for heat recycling, the gas undergoes further dust removal through a bag-type dust remover and a venture-type dust remover and CO2 removal through a CO2 remover to yield clean coal gas for recycling as the fluidizing and reducing medium. Lurgi established a pilot scale test platform of a circulating fluidized bed with a diameter of 700 mm and an output of 5t/d for developing the Cirfofer technology. Until 2003, over ten rounds of tests have been conducted with a total running time of more than 70 days. However, from reports in existing documents, the Circofer technology only passed the above pilot scale test, without further reported pilot scale test or industrialized application.
In the HISmelt process, a transport bed, in combination with a four-stage cyclone preheater, preheats the iron ore powder before entering into a smelting and separating furnace (http://www.hismelt.com; Schenk et al., Particuology, 2011, 914-23). As it essentially does not belong to iron ore reduction technologies, and is not in close association with the present application, this technology is not detailed herein.
Except for the above fluidized-bed reduction processes which either have undergone pilot scale tests or further have been industrialized, many patents relating to the processes for reducing iron ore on fluidized beds have been filed home and aboard. In these patents, 2- to 4-stage fluidized beds are generally used for reduction, which is similar to the FIOR/FINMET and FINMET processes. Some of the processes are even essentially the same as the above two processes, whereas others differ in different combinations of stages of fluidized beds, powder preheating mode, gas preheating mode, gas operation mode, etc., thus forming a number of granted patents. It also indicates from one aspect that there is still a huge innovation space for the iron ore fluidized reduction process from the different combinations of the main aspects of stages of fluidized beds, iron powder preheating, reducing gas preheating and gas operation mode. The iron ore fluidized reduction processes home and abroad are analyzed from the above respects as follows.
1) Stages of fluidized beds: four-stage fluidized beds are used in the mainstream FIOR/FINMET and FINEX processes. Patents that employ similar four-stage fluidized beds for reduction also include: US20120328465 (2012), CN101892339 (2012), CN101397597 (2010), CN101519707 (2010), CN100560739 (2009), US20080277842 (2008), AU2001265669 (2001), etc., wherein their iron ore powder reduction parts are essentially the same as those of FIOR/FINMET and FINEX. The patents such as CN103221555 (2013), CN102127611 (2012), U.S. Pat. No. 6,960,238 (2005), U.S. Pat. No. 6,736,876 (2004), US20020166412 (2002), and U.S. Pat. No. 5,785,733 (1998) employ three-stage fluidized beds for reduction. Except for the Lurgi's process, CN201563469 (2010), CN101333575 (2010), and CN101906501 (2010) relate to two-stage fluidized beds for reduction.
2) Powder preheating: In the Lurgi's process, a combined circulating fluidized bed-cyclone preheater-venturi unit is used. In CN101906501 (2010), a five-stage cyclone preheater is used to preheat iron ore powder. In CN101333575 (2010), a slope furnace is used to preheat iron ore powder. In other patents without specific iron ore powder preheating unit, the last stage fluidized bed functions to preheat the iron ore powder, which is, in fact, essentially similar to those of the FIOR/FINMET and FINEX processes.
3) Gas preheating: a gas preheating unit is set up in the Lurgi's fluidized-bed reduction process, whereas in many patents including the patents relating to the FIOR/FINMET process (U.S. Pat. Nos. 5,082,251, and 5,192,486) and the FINEX process (U.S. Pat. Nos. 5,762,681, and 5,785,733), no preheating unit is involved or contained. However, if the gas is not preheated, the temperature of the fluidized bed cannot be maintained at 800° C. or above, which is required for the reaction. In some processes including those in patents such as CN10151970 (2010), CN101906501 (2010), AU2001265669 (2001), and U.S. Pat. No. 6,736,876 (2004), the hot gas discharged from a melting gasifier is introduced directly into the final stage fluidized bed so that the sensible heat of the hot gas from the melting gasifier can be directly utilized. However, the reducing capability of the hot gas would be weakened if the gases generated during the melting and separating process, such as CO2, H2O, etc., are not removed. Actually, a gas preheating unit is set up in the actual flow of FIOR/FINMET (Schenk et al., Particuology, 2011, 914-23), i.e., the reducing gas is preheated by a preheater before passing through the fourth stage fluidized bed, the third stage fluidized bed, the second stage fluidized bed and the first stage fluidized bed sequentially. As the reaction process absorbs heat in general, when no preheater is set up thereamong, the temperature of the fluidized bed falls gradually, reaching as low as 400-500° C. in the last stage fluidized bed (the first fluidized bed), resulting in poor reducing capabilities. To resolve this problem, U.S. Pat. No. 6,960,238 (2005) suggests that oxygen/air is introduced into hot gas before the gas enters into each fluidized bed so that the reducing gas would be directly heated up through its partially combustion. Though the temperature of the gas is elevated by direct partial combustion, the CO2 and H2O generated from combustion would greatly lower the reduction potential and weaken the reducing capability of the gas, which is quite adverse to the reduction process.
4) Gas operation mode: in all existing processes, gas is operated in series and the reaction pressures vary significantly. The operating gage pressure is 11-13 atm for FIOR/FINMET; 2-4 atm. for FINEX; and 4 atm. for Circored. The operating gage pressure in CN100560739 is 4-10 atm. In some patents, such as CN101519707 (2010) and CN102127611 (2012), an operating pressure of 1-10 atm is provided. However, it is not operationally practical due to the wide range, because ten times of difference between the highest and the lowest operating pressures means the same ten times of difference between the highest and the lowest linear speeds of the fluidized beds, and generally it is hard for fluidized beds to have such operational flexibility. In many other patents, such as CN101333575 (2010), 101563469 (2010), CN103221555 (2013), and CN101892339 (2012), etc., the operating pressure is not described.
Although compared to a shaft furnace, the fluidized bed has many advantages including good contacting between the gas phase and solid phase, high heat and mass transfer efficiency, etc., the energy consumption for producing hot briquetted iron by the FIOR/FINMET process is about 15.0 GJ per ton, which is much higher than 10.5 GJ of the MIDREX process based on the shaft furnace, suggesting that the processes based on the fluidized bed still have much room for improvement in reducing energy consumption. Existing processes for reducing iron ore powder on fluidized beds still have the following two respects of problems.
High operational pressure: in both FIOR/FINMET and FINEX processes, high operating pressure is used (for example, the operating pressure in FIOR/FINMET is 12-14 atm). Since the gas compression process is energy-intensive, the gas compression consumption would be lowered greatly if the operating pressure can decrease to near atmosphere pressure (the operating pressure in MIDREX is 1-1.5 atm), and in turn the efficiency of fluidized-bed reduction of iron ore would be improved. One of the main reasons of using high-pressure operation in existing processes is that the diameter of the fluidized bed reactor can be reduced thereby. Since reducing iron ore to metallic iron by gas demands highly in thermodynamics, the iron trioxide in iron ore can be reduced to metallic iron only when the gas is excessive in a large amount. Therefore, a large amount of gas is required in the reduction process. An operation under the atmospheric pressure usually requires an oversized diameter of the fluidized bed. For example, in a 1-million-ton system using the FINMET process with an operating pressure of 12-14 atm, the diameter of the fluidized bed is still 5 m. It can be calculated that if the operation is conducted under the atmospheric pressure, the diameter of the fluidized bed will reach 17.7 m.
(2) low reduction efficiency: Although most processes use four-stage fluidized beds for reduction, no intermediary heat supplementation for the gas passing from the fourth stage to the first stage fluidized beds in series results in gradual reduction of the reaction temperature from the fourth stage to the first stage, with the temperature in the first-stage fluidized bed of only 400-500° C. Since iron ore exhibits slow reduction kinetics under a temperature below 700° C., the last two stages of the conventional four-stage fluidized beds play a minor role in reduction, resulting in a low overall reduction efficiency.
In summary, lowering operating pressure of the fluidized beds and increasing the overall reduction efficiency of the multi-stage fluidized beds through process and technical innovation are the key for reducing energy consumption during iron ore reduction on fluidized beds and improving economic efficiency of the reduction process.